On-demand vapor generator and control system

ABSTRACT

The disclosed apparatus and control system produces a single, on demand, energetic gaseous working fluid from any heat source. Working fluid in a liquid phase is released into a heat exchange tube in the form of very fine droplets or atomized mist, where it is rapidly heated to its gaseous phase. The gaseous working fluid can continue to absorb heat before exiting the heat exchange tube to perform work. The disclosed system controls the release of working fluid into the heat exchange tube and/or the heat energy to which the tube is exposed, resulting in a flow of energetic gaseous working fluid that can be quickly adjusted in response to changing conditions without a large pressure vessel.

BACKGROUND

The disclosure relates to devices and methods for generating controlled flows of energetic working fluids without a pressure vessel.

Phase-change working fluids are commonly used to translate heat energy into useful work or to transport heat from one location to another. The use of water as a working fluid where heat energy is absorbed to change water into steam that is then used to perform work is well-known. Other working fluids such as refrigerants may be employed in a similar fashion. Many prior art energy translation systems employ a boiler to heat large quantities of water to generate a flow of steam that can then be used to move heat energy from the boiler to another location (as in steam heat systems) or to perform work (as in steam engines, turbines, etc.). The reserve of steam available in a pressure vessel allows systems to respond quickly to changes in demand through controlled release of steam from the pressure vessel. The energy stored in such pressure vessels presents significant hazards and may require special siting permits from state and local governments as well as dedicated staff to monitor and operate such systems.

There is a need for a non-boiler steam source that is capable of providing a consistent flow of steam that is responsive to changing conditions.

SUMMARY

The disclosed apparatus and control system produces a single, on demand, energetic gaseous working fluid from any heat source. Working fluid in a liquid phase is released into a heat exchange tube where it is heated to its gaseous phase. The gaseous working fluid can continue to absorb heat before exiting the heat exchange tube to perform work. The disclosed system controls the release of working fluid into the heat exchange tube and the heat energy to which the tube is exposed, resulting in a flow of energetic gaseous phase working fluid that can be quickly adjusted in response to changing conditions without a large pressure vessel. As used in the context of this application, “heat exchange tube” means a structure having a large surface area relative to its internal volume to facilitate heat exchange between a source of heat energy and the working fluid inside the heat exchange tube. It is important to note that a heat exchange tube in the context of the present invention does not contain a significant volume of liquid phase working fluid. Liquid phase working fluid is released into the heat exchange tube in concert with the application of heat to the heat exchange tube and at a rate ensuring that liquid phase working fluid does not accumulate in the heat exchange tube. Heat exchange tube is not limited to an elongated tubular shape in the traditional sense, and should be interpreted broadly according to the definition given above. Whatever the shape of the heat exchange tube, it is necessary that the tube be capable of containing the working fluid during transition from liquid to gas and also to contain the gaseous working fluid up to the maximum working pressure of the system, including a margin of safety.

Heat may be generated by means of external combustion from any fuel such as natural gas, wood, garbage, etc. External combustion has many advantages relative to internal combustion, with the main advantage being that such systems are fuel agnostic. Any fuel that provides heat can be used to produce work in conjunction with the disclosed apparatus and control systems, whereas internal combustion requires a specifically refined fuel. The disclosed apparatus and control systems are not limited to external combustion as a source of heat. The disclosed concepts are compatible with any form of heat, including but not limited to waste heat in the exhaust from internal combustion engines, furnaces, kilns, etc. The term “heat exchanger” as used in this application applies to any arrangement configured to facilitate movement of heat from one fluid to another and includes liquid to gas, liquid to liquid, and gas to gas heat exchangers. Heat exchangers are configured for specific installed environments and take many forms that are well-understood to those skilled in the art, all of which are encompassed by the term “heat exchanger” as used in this application.

The disclosed embodiments employ water as the working fluid, but the apparatus, methods and concepts are not limited to only water as a working fluid. Other mediums have been used for two-phase vapor-liquid power systems, including but not limited to ammonia, refrigerants, isobutane, and isopentane. The Organic Rankine Cycle (ORC) is named for its use of an organic, high molecular mass fluid with a liquid-vapor phase change, or boiling point, occurring at a lower temperature than the water-steam phase change. Organic fluids compatible with ORC systems may be used as working fluid in systems constructed according to this disclosure. Any fluid with a phase change from a liquid to a gas by means of heat transfer may be compatible with the present disclosure, however water steam has properties that are particularly beneficial in the context of the disclosure systems and methods.

The gaseous phase of the working fluid is employed as an energetic fluid to do work (for example, to generate electricity) through an expander (turbine, piston engine, etc.) or to produce a controlled flow of heated gaseous phase working fluid such as steam for process applications. Past methods for generating a controlled flow of steam require a boiler which accumulates steam in a pressure vessel and releases steam from the pressure vessel as desired to perform work. Maintaining a reservoir of steam means that additional demand can be quickly met by releasing additional steam from the reservoir. However maintaining a large reservoir of pressurized steam is less efficient than generating steam on-demand in response to changing system conditions. Systems employing even a modest size reservoir of liquid phase working fluid can require substantial time before any gaseous phase working fluid is available, typically requiring that the reservoir remain heated even when there is no demand for gaseous phase working fluid. Additionally, a large reservoir of steam can present safety risks that add to the cost and complexity of such systems.

The present disclosure relates to a system that generates on demand gaseous phase working fluid from controlled release of liquid phase working fluid into a heat exchanger, and a control system to regulate the flow of liquid phase working fluid and the heat energy to which the working fluid is exposed. The resulting flow of gaseous phase working fluid can be controlled in terms of its quantity, temperature and pressure and used to perform a desired work or process.

Due to the very large expansion of water when it becomes steam, the volume and properties of the steam generated in such a system will tend to fluctuate, which is undesirable. An electronic control system regulates the flow of water (liquid phase working fluid) into the heat exchanger and the heat energy applied to the heat exchanger to produce a steady flow of steam at a desired temperature and pressure. Information from temperature and/or pressure sensors at the outlet of the heat exchanger is employed by the control system to stabilize the steam output in a feedback arrangement. In addition to generating a steady state flow of steam, embodiments of disclosed systems and control methods are capable of increasing or decreasing the flow of steam in response to system inputs calling for decreased or increased work. The control system regulates the mass flow of water and the heat delivered to the heat exchanger, which allows the system to fluctuate to any desired power output (within equipment limits).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a basic on demand steam generating apparatus and control system according to aspects of the disclosure;

FIG. 2 is a block diagram of an on demand steam generating system arranged to generate electricity and supplementary heat in a combined heat and power arrangement according to aspects of the disclosure;

FIG. 3 is a perspective view of a heat exchanger showing input and output couplings according to aspects of the disclosure;

FIG. 4 is a schematic showing components of a representative controller for use in the disclosed systems and methods;

FIG. 5 is a flow chart illustrating exemplary program steps executed at system start up according to aspects of disclosed methods;

FIG. 6 is a flow chart illustrating alternative program steps executed at system start up according to aspects of disclosed methods;

FIG. 7 is a flow chart illustrating exemplary program steps executed during steady state operation according to aspects of the disclosed methods;

FIG. 8 is a flow chart illustrating alternative program steps executed during steady state operation according to aspects of the disclosed methods;

FIG. 9 is a flow chart illustrating exemplary program steps executed during steady state operation of the disclosed system according to aspects of the disclosure; and

FIG. 10 is a simplified example of a heat exchange tube according to aspects of the disclosure.

DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENTS

Disclosed systems and methods will be described using water as the liquid phase working fluid and steam as the gaseous phase working fluid. Those skilled in the art will recognize that other working fluids may be employed and that the disclosed systems and methods are not limited to a water/steam working fluid.

FIG. 1 illustrates a basic on demand steam generating system according to aspects of the disclosure. The system 100 includes a water tank 1, water pump 3, check valve 4 and nozzle 5 arranged to deliver water into a heat exchanger 6 within a combustion chamber 7. Liquid water 2 is pressurized by water pump 3 to produce a pressure differential across nozzle 5, resulting in liquid water entering the heat exchanger 6. The water delivered to the heat exchanger 6 absorbs heat energy sufficient to change phase to steam, expanding rapidly as a result. The steam continues to absorb heat as it fills the one or more heat exchange tubes 30 in the heat exchanger 6. In one disclosed embodiment, the heat exchanger 6 and combustion chamber 7 may be similar to that employed in a natural gas condensing boiler, with one or more coiled heat exchange tubes 30 arranged in a combustion chamber 7 such as that illustrated in FIG. 3. The nozzle 5 is arranged at or near an otherwise closed end of the heat exchange tube 30. As injected water transitions to steam, it expands to greater than 1000 times the liquid volume, filling the tube with steam. As newly injected water is converted to steam, new steam displaces steam already in the tube 30, producing a flow of steam 10 away from the nozzle 5. The length of the tube 30 into which the water is injected allows the steam to continue to absorb heat as it moves away from the nozzle 5, becoming dry, superheated steam that is ideal for use in a variety of expanders to perform work. Pressure and/or temperature sensors 8, 9 are arranged at the outlet of the heat exchanger 6 and provide corresponding pressure P and temperature TEMP signals to a controller 11.

The controller 11 is a programmable device with inputs, outputs, memory and a processor, as shown in FIG. 4. The controller may be a microcontroller and include a communication interface allowing a computer or other input/output device to be connected for programming and monitoring the microcontroller during operation as is known in the art. The controller 11 is programmed with one or more sets of program instructions that are stored in memory. Data is also stored in memory and may be used during operation of the on-demand vapor generator 100. The controller 11 may be configured to cooperate with a burner controller 24, provided by the manufacturer of the burner 12 to control the quantity of heat released into the combustion chamber 7. The controller 11 adjusts operation of water pump 3 to control the mass flow of water injected into the heat exchange tube 30. Assuming the presence of sufficient heat energy to convert injected water into steam, increased differential pressure across nozzle 5 results in increased mass flow of water into heat exchange tube 30 and increased volume of steam 10 at the outlet of the heat exchanger 6. If the heat energy remains constant, increased mass flow of water results in a decrease in temperature of the steam at the outlet of the heat exchanger 6. As discussed in greater detail below, control methods may vary fluid flow, heat input, or both to provide a stable flow of gaseous phase working fluid (vapor).

According to aspects of the disclosure, the pump 3 may be driven by a brushless direct current (BLDC) motor 32 in which the torque generated by the motor is proportional to the applied power and is adjustable using pulse width modulation (PWM). The controller 11 adjusts the power applied to the pump motor 32 to vary the pressure differential across the nozzle 5, which regulates the mass flow of water through the nozzle 5. The relationship between power applied to the water pump motor 32 and the mass flow of water through nozzle 5 can be established by experimentation and stored in memory for use in the controller program, resulting in predictable delivery of water into the heat exchanger 6. In an alternative arrangement, a mass flow sensor can be arranged to detect the flow of water through the nozzle 5 and a signal corresponding to the measured mass flow employed by the controller to adjust operation of the water pump motor. Heat output of the burner 12 can be similarly established by operating the burner over a range of settings to establish control signals to the burner 12 corresponding to quantities of heat energy released into the combustion chamber.

FIG. 10 illustrates a simple heat exchange tube having a circular cross section. Any heat exchange tube configuration is compatible with the disclosed system, so long as the tube has sufficient strength to contain the steam at the pressure level needed by the expander attached to the output of the heat exchanger 6. One example of an expander is a steam turbine driven generator, which requires approximately 0.004 Kg per second of superheated dry steam at a temperature of approximately 350°-400° F. and pressure of about 75 psi. Steam that is too cool and wet or steam that is too hot will result in damage to the turbine 15, so control of the steam parameters of temperature and pressure is needed.

Heat may be generated by a burner 12 combusting fuel under the direction of the controller 11, 24 as shown in FIG. 2, in which case the controller 11 is configured to cooperate with the burner controller 24 to fire the burner to pre-heat the chamber 7 surrounding the heat exchanger 6. In some embodiments of the disclosed systems 100, 200, the burner 12 is a burner configured for use in a residential boiler or heating system, and is preprogrammed with safety checks to confirm safe operation of the burner 12. Safe operation of the burner 12 is reported to the system controller 11 at startup by the burner controller 24. It is an objective of the control system that the water absorbs sufficient energy and phase change to steam before it can collect as water droplets on the inside wall of the heat exchange tube. If water drops collect in liquid form, they can then pool inside the heat exchanger and cause system instabilities and inefficiencies. In one embodiment of a heat exchange tube 30 as shown in FIG. 10, water is injected at the top end of a helical coil, and the water flows down the coil, spreading and absorbing heat. Situating the nozzle 5 in a position where gravity assists in spreading the water along the heat exchange tube 30 may prevent pooling of liquid phase working fluid. As steam moves along the remainder of the coiled tube 30 as a gas, it absorbs more heat, and becomes dry super-heated steam.

FIG. 2 illustrates one embodiment of an on demand steam generating system 100 incorporated into a combined heat and power (CHP) system 200. The basic system 100 of a working fluid reservoir 1, water pump 3 driven by BLDC motor 32, check valve 4, nozzle 5, heat exchanger 6, combustion chamber 7, burner 12, sensors 8, 9 and controller 11 operate in a manner described above with respect to FIG. 1. The basic system 100 is arranged to deliver steam to a turbine-powered electrical generator 15 to generate electrical energy. A coolant pump 21 circulates coolant 23 through an exhaust gas heat exchanger 19 and a condenser 17 to collect heat energy from the heated combustion (exhaust) gasses leaving the combustion chamber and from the low pressure steam leaving the exhaust of the steam turbine 15, respectively. Coolant 23 is circulated through a heat exchanger 20 and the heat harvested from the condenser 17 and exhaust gas heat exchanger 19 is used to satisfy a heat load such as heating a building or for some other purpose, increasing the overall efficiency of the CHP system 200.

In the disclosed system 200, the water pump motor 32 and the burner blower motor 34 are both variable output motors whose output is proportional to applied power. In the case of the burner blower motor 34, the burner 12 is configured to meter fuel according to the power applied to the blower motor 34, so in the disclosed system 200, power applied to the burner blower motor 34 is a proxy for heat energy input to the combustion chamber 7. Power applied to the water pump motor 32 and burner blower motor 34 is varied using pulse width modulation (PWM) as is known in the art. Other methods of motor control may be employed and the disclosed control scheme is intended as a non-limiting example.

The disclosed system 200 is a “closed loop” working fluid system that is sealed, eliminating the need for added “make up” working fluid. Once assembled and before being sealed, the water tank 1, water pump 3, heat exchanger 6, turbine 15, condenser 1 and associated fluid flow conduits are evacuated and then the water tank 1 is filled, or “charged” with a pre-determined volume of de-gassed, distilled water. The system 200 is intended to remain sealed and operate using the same quantity of working fluid, but can be unsealed for service as necessary. Evacuating and sealing the system 200 ensures that the system is filled with only water and water vapor, and eliminates the need for a vacuum pump (such as 18 shown in FIG. 2) to maintain low pressure at the outlet of the turbine driving the electrical generator 15. Low pressure steam 16 leaving the turbine 15 is cooled and condensed in condenser 17 back to liquid water and returned to the coolant reservoir 1. Use of distilled water as a working fluid can reduce fouling and build up in the heat exchange tubes 30 due to dissolved minerals present in most water. Further, re-use of the working fluid 2, 10, 16 means that any residual heat left after the turbine exhaust 16 is condensed is heat energy that does not need to be added to convert the liquid water back to steam, which increases the efficiency of the CHP system 200.

The combustion chamber 7 is an area where heat is contained and concentrated, facilitating transfer of heat to the heat exchange tube or tubes 30 and the water and steam within the tube. The chamber 7, which may be referred to as “the combustion chamber,” is not limited to combustion only. Heat may be provided from an outside source, in which case the combustion chamber 7 provides an environment for the heat to surround and raise the temperature of the heat exchanger 6. The heat exchanger 6 is a device which facilitates the transfer of heat energy to the working fluid to phase change from liquid to gas and accumulate pressure. In the embodiment illustrated in FIG. 2, the heat exchanger 6 is one or more tubular metal coils 30 such as that illustrated in FIG. 10, which can withstand the temperatures and pressures of the combustion chamber and the gaseous working fluid 10. The geometry and material of the heat exchanger 6 and heat exchange tubes 30 can vary depending upon specific variables, including but not limited to: the rate at which heat is transferred from the chamber 7 to the working fluid in the heat exchange tube(s) 30, the volume, pressure and temperature of the gaseous working fluid output, spatial constraints, and economic factors such as material costs and manufacturing considerations. The heat exchanger 6 can be, but is not limited to a circular cross section metal coil, a finned tube, flat tube, or shell and tube heat exchanger, etc. A heat exchange tube in the form of a coil surrounding a vertical axis may have the nozzle 5 for delivering liquid phase working fluid arranged at an upper end of the tube. Injecting liquid phase working fluid (water) at an upper end of the tube 30 allows the liquid phase working fluid to flow and spread along the tube by gravity to enhance heat transfer from the tube to the working fluid.

A controller 11 compatible with the disclosed on-demand vapor generator is operatively connected to the burner 12, working fluid pump 3, and sensors 8, 9 arranged to sense various conditions in the vapor generator 100, 200. As shown in FIG. 4, a controller 11 includes a processor (CPU), memory, inputs, outputs, a power supply and a communication interface for installing or changing the operating program. The controller 11 is configured to cooperate with a controller 24 for the burner 12, allowing the use of an “off the shelf” burner available for residential or commercial boilers. Commercially available burners meet regulatory requirements for safety, so configuring the systems 100, 200 to use commercially available burners simplifies regulatory compliance. Some parts of the description may refer to “controller 11, 24” meaning that the system controller 11 is configured to coordinate with the burner controller 24 to manage the generation of heat for the system 100, 200. A suitable controller can be constructed using a computer or a microcontroller or microprocessor and memory according to the preference of a designer. An operating program, to be executed by the controllers 11, 24 may be one or more sets of code executed separately in controllers 11 and 24, with operation of the burner controller 24 responsive to commands from the system controller 11. The system firmware may include one or more program loops such as those illustrated in FIGS. 5-9. Primary objectives of the controller are to: 1) manage cold start-up of the system to quickly achieve a stable steam flow output without human intervention; 2) monitor system operation, making adjustments as necessary to maintain stable steam production; 3) adjust system operation in response to changes in demand for steam; and 4) take corrective action or shut down the system in the event of an unsafe or unstable condition.

Sensed conditions internal to the vapor generator include temperature TEMP and pressure P in the heat exchange tube 30. The control system includes a sequence for starting the disclosed on-demand vapor generator from a cold state. One start up sequence illustrated in FIG. 5 includes operating the burner 12 and pump 3 according to a pre-determined “initial” set point while monitoring temperature TEMP at or near the outlet of the heat exchanger 6. The initial set point includes an initial heat set point and an initial fluid flow set point corresponding to a pre-determined flow of gaseous phase working fluid at a pre-determined temperature and pressure. The initial set point and start sequence shown in FIGS. 5 and 6 are selected to safely manage the system from a cold start to steady state operation at a low rate of vapor production. When the system has achieved steady state operation, the controller is programmed to shift to the steady state control loops shown in FIGS. 7, 8 and 9.

According to the algorithm illustrated in FIG. 5, the controller 11 first conducts some safety checks to ensure that there are no errors from the burner controller 24, sufficient water is present in the water reservoir 1, etc. If there are no safety issues, the controller 11 operates the burner 12 at an initial heat set point and the pump 3 at an initial fluid flow set point, where the burner 12 may be started before the pump 3 to allow the combustion chamber 7 and heat exchanger 6 to warm up. The controller 11 monitors the temperature TEMP at the outlet of the heat exchanger 6 and adjusts operation of either the pump 3 or the fuel burner 12 while operating the other of the pump 3 or fuel burner 12 at its initial setting. As shown in FIGS. 5 and 6, the controller 11 compares a sensed condition such as temperature TEMP with a desired level T for the sensed variable and determines the difference between sensed condition TEMP and the desired level T, where the difference ΔT can be positive when the sensed condition TEMP is greater than the desired level T or negative when the sensed condition TEMP is less than the desired level T. It will be noted that ΔT is calculated differently in FIGS. 5 and 6 to account for the fact that, when controlling the variable TEMP, increased fluid flow acts to reduce TEMP, while increased heat energy acts to increase TEMP. The controller 11 is programmed to apply a correction factor to operation of the fuel burner 12 or pump 3, where the correction factor is a combination of a pre-determined constant multiplied by the difference between the sensed condition and the desired level, or in this example ΔT. The controller 11 is programmed to monitor the sensed condition and adjust operation of the fuel burner 12 or pump 3 until the sensed condition remains within a pre-determined range for a pre-determined period of time. If the sensed condition is temperature T in the heat exchanger, then one example of a pre-determined range might be a range of +/−15° F. for a period of 20 to 40 seconds. FIG. 5 illustrates a startup algorithm where the burner is operated at the initial set point and fluid flow is modulated by applying a fluid flow correction factor to the power applied to the pump motor 32. According to aspects of the disclosure, the constant X used in the fluid flow correction factor is very small, for example 0.05. The resulting fluid flow correction factor will vary with ΔT, but will always be small. Small adjustments made many times per second allow the disclosed controller to quickly stabilize the variable TEMP within the desired range without large swings or oscillations. The constant X may be different in the steady state algorithm of FIG. 7. One representative example might be 0.02. The value of constant X in the startup algorithm of FIG. 5 is larger because the circumstances call for greater change in TEMP during startup, while the smaller constant should be sufficient for steady state operations where TEMP does not need to change as much.

FIG. 6 illustrates a startup algorithm where the pump 3 is operated at the initial set point and the burner is modulated by applying a heat correction factor applied to the control signal from the controller 11 to the burner 12, where the control signal may be power varied using PWM. The constant Y used in calculating the heat correction factor has a first value Y1 when T is greater than TEMP (calling for reduced heat input) and a second value Y2 when T is less than TEMP (calling for increased heat input). To reduce the chance of temperature overshoot, Y1 is greater than Y2. Constants Y1 and Y2 may also be smaller during steady state operation when heat input is modulated to control TEMP, than during startup.

FIG. 7 illustrates a steady state control loop where the system 200 is operated at a set point corresponding to current demand for the combined output of the system, e.g., electricity and/or heat. In FIG. 7, heat output is held at the demand set point and water flow is modulated by applying a steady state fluid flow correction factor to the power applied to the pump motor 32. The variable TEMP is compared to the desired temperature T and the steady state fluid flow correction factor is calculated and the pump operation is adjusted accordingly. Note that the constant X used in calculating the fluid flow correction factor may be different in the steady state control loop of FIG. 7 than it is in the startup control loop of FIG. 5. In FIG. 7, a change in demand causes the controller 11 to look up a new set point corresponding to the new demand. The pump and burner are operated according to the new set points, with water flow varied by application of the fluid flow correction factor calculated as discussed above. Water flow is varied at the new set point to control TEMP within a predetermined range of the desired temperature T until there is another change in demand. The steady state control algorithm of FIG. 7 would be used in a system configured to use water flow to control steam temperature, and is compatible with the startup algorithm of FIG. 5. FIG. 8 illustrates a steady state control algorithm that modulates heat output to control steam temperature TEMP, and is compatible with the startup algorithm of FIG. 6. Constant Y may be smaller in the steady state control algorithm of FIG. 8 than in the startup control algorithm of FIG. 6.

According to aspects of the disclosure, when the hardware of an on-demand vapor generator 100 has been selected and assembled into a CHP system 200, the vapor generator 100 is operated to characterize or map behavior of the vapor generator 100 over a range of operating conditions. For example, the pump 3 is operated at a set point and the fuel burner 12 is operated over a range of settings to determine a heat setting which results in a steady state flow of gaseous phase working fluid for the flow of liquid phase working fluid corresponding to the pump flow rate. Steady state as used in this disclosure is a flow of gaseous phase working fluid that remains within a 40° F. temperature range (+/−20° F. of a target temperature T) and within a 10% range of gaseous phase working fluid pressure for a pre-determined period of time. The pre-determined period of time may range from 10 seconds to one minute, depending upon system configuration. This process is repeated at a range of pump settings corresponding to expected operating conditions of the on-demand vapor generator, a corresponding heat setting is determined for each pump setting, and the pump settings and heat settings are saved into memory as “set points” each including a pump setting and a heat setting. This process can be repeated with the fuel burner 12 operated at a range of heat settings corresponding to expected operating conditions of the on-demand vapor generator 100, and the corresponding pump setting for each heat setting is established and saved into memory. This process may result in one or more “maps” relating fluid flow to heat input for a specific CHP system configuration. At least one of these “maps” is saved into the memory of the controller for use in control algorithms such as those illustrated in FIGS. 5-8. When the vapor generator 100 is incorporated into a CHP system 200, the pump and heat settings may correspond to the expected range of heat and electrical output from a “startup” setting to settings corresponding to maximum output. In this disclosed control method, one of the pump or burner are operated at a fixed setting and the other of the pump or burner are modulated to determine the other part of the control set point. In the disclosed embodiment, the set points correspond to a duty cycle (PWM signal corresponding to applied power) to be applied to the water pump motor 32 and the burner blower motor 34. Power applied to water pump motor 32 corresponds to line pressure at the nozzle 5 and a mass flow of water through the nozzle (which will vary according to back pressure). Power applied to burner blower motor 34 corresponds to heat energy released into combustion chamber 7, as described above. The purpose of the initial set point is to give the controller 11, 24 initial operating parameters until system parameters stabilize, allowing a shift to the steady state algorithms of FIG. 7, 8 or 9.

As the steam 10 leaves the heat exchanger 6, a temperature sensor 9 such as a thermocouple and/or a pressure sensor 8, such as a pressure transducer read the steam parameters (temperature and pressure) and relay corresponding pressure and/or temperature signals to the controller 11. Multiple temperature and/or pressure sensors may be used to improve the accuracy or detail of sensed conditions in the system 100, 200. The disclosed systems 100, 200 may take 40 readings per second and average the readings over a one second period to arrive at a measured value for temperature and pressure. Averaging multiple sensor readings can reduce the effect of erroneous readings. Other sensors, such as a mass flow meter on the water pump 3 could be employed to provide information to the controller 11.

Water transitions to steam and expands rapidly, increasing the pressure in the heat exchange tube 30 adjacent the nozzle 5. As the steam 10 fills the heat exchange tube and connected steam lines, the pressure within the heat exchange tube 30 increases, resulting in higher pressure opposing the injection of water from the nozzle 5. Changes in differential pressure across the nozzle 5 result in changes in the flow of water through the nozzle 5. The water pump motor 32, water pump 3, nozzle 5, and heat exchange tubes 30 are selected so that the water pump 3 can generate sufficient differential pressure across the nozzle 5 to overcome increased back pressure from steam being generated in the heat exchange tube 30. Increased differential pressure across the nozzle 5 results in increased water injection and increased steam flow, assuming sufficient heat energy to transition the injected water 2 to steam 10. Decreased differential pressure reduces water flow through the nozzle 5, resulting in a reduced demand for heat energy to transition the reduced flow of water 2 into steam 10, which requires adjustment of the burner 12 to prevent overheating. The water pump motor 32 is selected to have the capability of generating pressure at the nozzle 5 sufficient to overcome the back pressure in the heat exchange tube 30 at a pre-determined maximum steam output pressure. An alternative design could employ an active nozzle that would respond to signals from a controller to turn on, turn off or modulate flow through the nozzle. Such a nozzle would present control modalities in addition to those discussed in the present embodiments. Different arrangements of hardware can be operated and mapped as described above. Alternative system configurations will have operating parameters dictated by component configurations and operating requirements.

One example of initial set point values are a 25% duty cycle applied to the burner blower motor 34 and a duty cycle applied to the water pump motor 32 sufficient to produce about 4.6 mL/sec. water flow through nozzle 5. One example of an equation for water flow is: 5.0166*Ln(Pressure)−15.111, which was derived by measuring water flow needed to keep the steam temperature at 400° F. at multiple pressures, then adding a best fit curve. These initial set point values consistently bring the steam pressure up to 50 psi without overheating the system. These values are employed in a system using only proportional control equations. One factor affecting system stability is a lag in temperature readings from thermocouple-type temperature sensors. In a system using proportional control, it is possible to add a correction factor to the duty cycle applied to the water pump motor 32 to minimize temperature overshoot caused by thermocouple lag. One example of a correction factor is a value of 0.015*(TEMP−400), with TEMP being the thermocouple reading and 400 being the desired stable temperature of output steam. The resulting correction is positive when thermocouple readings are greater than 400° F. and negative when the readings are less than 400° F. Use of proportional-integral-derivative (PID) control as shown in FIG. 9, is one alternative to the control algorithms of FIGS. 5-8.

Pressure control is similar to water flow control but is linked to the duty cycle applied to the burner blower motor 34, since the energy released into the combustion chamber is directly related to the burner blower motor duty cycle. A pressure transducer 9 measures pressure before the turbine and takes an average of multiple measurements. The average measured pressure is compared to the desired pressure and a heat correction factor as shown in FIG. 6 is applied to the blower motor duty cycle. The system 100, 200 may use different constants when calculating a correction factor to reduce the likelihood of temperature overshoot. For example, during startup when the average measured value is less than the desired pressure (calling for increased heat input), the correction constant is 0.007, and when the average measured value is greater than the desired pressure (calling for reduced heat input), the correction constant is 0.015. During steady state operation, the corresponding values are 0.01 and 0.015, respectively. In both cases, the correction constant is smaller for situations calling for increased heat than it is for situations calling for decreased heat.

In one example, the initial set points allow the system to safely reach 50 psi, at which point the blower motor duty cycle is reduced to 22%. Once the system drops to 43 psi at the 22% duty cycle, automatic control is enabled, allowing the blower motor duty cycle to slowly increase by using a proportional gain equation until steam pressure steadies at 50 psi. To verify the system is stable, pressure boundaries of 48.5 psi and 51.5 psi and temperature boundaries of 380° F. and 415° F. are established. Once the system stays within those boundaries for 20-40 seconds, the system is considered stable and is allowed to ramp up in pressure to the desired value. When ramping up steam pressure under proportional gain control, the set point may be increased by 1 psi increments, allowing the pressure to rise to within 1.5 psi of the set point for 2 seconds before increasing the set point by another 1 psi. Similar methods may be used during pressure decrease. Alternatively, when entering steady state control, a set point corresponding to the current demand may be selected, and system operation shifted to that set point as shown in FIGS. 7 and 8, rather than being ramped incrementally.

Valves 13 and 14 are arranged to control when steam is delivered to the turbine 15. During startup, the first steam produced will be too cool and wet to be used in the turbine, so valve 13 is open and valve 14 is closed, diverting steam 10 around the turbine 15 to condenser 17. When temperature and pressure signals from sensors 8 and 9 are within a range suitable for the turbine 15, controller 11 closes valve 13 and opens valve 14, routing steam through the turbine to perform work such as generating electricity. This would typically occur upon entering the steady state control loops at the conclusion of FIG. 5 or 6. Valves 13 and 14 could be replaced with a steam trap or a clean steam separator (not shown), which would provide the similar functionality as valves 13 and 14 operated by controller 11. The system 200 and controller 11, 24 are configured to protect the micro steam turbine from input steam that could damage the turbine. Steam that is too cool and wet or steam that is too hot will damage the turbine, so the system 200 and controller 11, 24 are configured to produce superheated dry steam at an output temperature range of between 350° F. and 400° F., ideally approximately 400° F. Steam temperatures below 250° F. or above 450° F. represent the absolute boundaries of acceptable input steam for the micro steam turbine 15. The system 200 and controller 11, 24 are arranged to produce and deliver steam in the heart of the acceptable range and to prevent damage to the turbine from exposure to steam having properties outside the acceptable range.

To the steam generator 100, the attached expander appears as a flow restriction resisting flow of steam away from the nozzle end of the heat exchange tube 30. Except for an initial spin up of the turbine 15, the flow restriction should be fairly consistent, allowing the controller to enter a steady state control loop illustrated in FIGS. 7 and 8. One example of a steam turbine 15 requires a flow of super-heated 400° F. dry steam at approximately 75 psi, and the controller 11, 24 is configured to balance the mass flow of water with applied heat energy to generate sufficient steam volume to maintain the required output temperature and pressure against the flow restriction of the turbine 15. For any given expander, the volume of steam generated at the output of the heat exchanger 6 during operation is balanced against the volume of steam passing through the expander, resulting in a steam pressure at pressure sensor 9. Increased steam production will tend to increase pressure at sensor 9 and increase the resulting work at the expander 15, while reduced steam production has the opposite effect. Alternatively, valves 13, 14 could be employed to control the volume of steam delivered to the turbine 15, and the resulting work performed, e.g., electrical energy generated by the electrical generator driven by the turbine 15. As previously described, when sensors 8, 9 report the presence of steam temperature and pressure at the outlet of the heat exchanger 12 that are suitable for the turbine 15, controller 11 opens valve 14, closes valve 13 and enters a steady state control loop at 70.

An alternative example of a steady state control loop is illustrated in FIG. 9. In this example, the steady state control loop includes two overlapping subroutines 72, 74. The disclosed control loops 72, 74 incorporate proportional, integral, derivative (PID) calculations to guide actions of the controller, with the objective being a stable output of steam at the correct temperature and pressure. Use of PID calculations in control loops is well known to those skilled in the art and will not be described in detail here. PID control can prevent excessive correction (overshoot), which can lead to undesirable system oscillations and instability. Using the mass flow of water as derived from operation of the water pump 3 (as described above) and/or a mass flow sensor (not shown), the controller 11 can then calculate how much heat is needed so the water flow entering the heat exchange tube 30 can phase change to steam before leaving the heat exchanger 6. The controller 11, 24 balances the injection of water with the addition of heat energy to produce a steady flow of steam having the correct pressure and temperature for use in the turbine 15. In the control method of FIG. 9, an electronic feedback loop maintains a steady state pressurized steam flow “set point” by adjusting the mass flow of water injected as well as the heat energy available in the combustion chamber 7 in response to signals from sensors 8 and 9.

In an embodiment of the disclosed on demand steam generating systems employing the steady state control loop of FIG. 9, the injection of water and the application of heat interact to produce the desired flow of steam, so the steady state program loop employs subroutines 72, 74 which constantly adjust power applied to the burner blower motor 34 and the water pump motor 32. At block 72, if steam pressure is not at the set point, controller performs a PID calculation to arrive at a correction to the blower motor duty cycle at 76, which roughly corresponds to heat energy input to the combustion chamber 7. The blower motor duty cycle is updated with the calculated correction at 78. The controller then calculates the water pump motor duty cycle based on the pressure sensor reading at 80 and updates the water pump motor duty cycle at 82. The program loop then checks the steam temperature at 84. If the measured temperature is not correct, the controller performs a PID calculation to arrive at a correction to the water pump motor duty cycle at 86 and updates the duty cycle of the water pump motor at 88. At the end of subroutine one (steps 72 and 76-88) the control loop returns to the beginning and checks steam pressure again at 72. If steam pressure is correct, then steam temperature is checked at 74. If steam temperature is not at the set point, then the controller performs a PID calculation to arrive at a correction to the water pump motor duty cycle at 90 and updates the water pump motor duty cycle at 92. At the end of subroutine two (steps 74, 90 and 92) the control loop returns to the beginning and checks steam pressure at 72. The steady state control loop of FIG. 9 runs continuously during operation of the system 100, 200, repeating many thousands of times per second to maintain the required flow of steam at a predetermined temperature and pressure. While a control feedback loop with PID control calculations to stabilize steam output is described, the disclosed systems are not limited to this control scheme.

There are a variety of expanders that can be used to produce work from the flow of energetic gaseous working fluid produced by the disclosed systems, including a piston engine, Wankel engine, micro or large scale steam turbines, etc. Each expander may require specific system features, such as lubrication oil, downstream pressure drops to draw condensate through the expander, etc. The system illustrated in FIG. 2 employs a condensing micro steam turbine which requires no oil lubrication but does require negative pressure on the exhaust to draw out any condensate. This alleviates any back pressure to the turbine, providing maximum efficiency. The system configuration will change based upon the chosen expander and system functionality, with the disclosed on demand steam generator and control system operating according to the same basic principles.

The disclosed on demand vapor generating system 100, or CHP system 200 require a set point for work output. The work output set point is based upon the system's work output requirement, or “demand.” When the controller 11 recognizes the work output requirements either through a preprogrammed input, a user interface where a set point value can be programmed, or through electrical or mechanical load recognition features such as a thermostat or demand for electrical energy, the system will identify a set point for the desired work output. The flow of water and heat input are calculated and controlled to generate a flow of steam corresponding to the desired set point of work output. The disclosed systems 100, 200 include controllers 11, 24 programmed to respond to changes in load by establishing a new set point and adjusting operation of the system to generate steam output corresponding to the required work output, within system limitations. Steam output is matched to demand on a continuous basis, according to aspects of the disclosure.

Once the steam 10 begins to flow to an expander such as turbine 15, work output can be measured by many different variables (current, voltage, RPM, etc.). Work output at an expander may be measured by the controller and used to calculate the correct heat input and/or water mass flow required to maintain the corresponding steam production. System 200 is configured to monitor steam temperature and/or pressure at the outlet of the heat exchanger 6 and to maintain these parameters at values corresponding to a demand for work.

The disclosed CHP system 200 will not only generate electricity but also produce residual heat that can be recovered as useful heat for hot water, facility heat, or other uses as shown in FIG. 2. In a CHP configuration such as that shown in FIG. 2, the overall system efficiency can be increased to over 95% and as high as 99% by recovering the normally wasted residual heat for other uses (System efficiency=(Electricity out+Heat recovered)/Fuel burn). In FIG. 2, the heat exchanger 20 arranged to remove heat from the coolant 23 circulating in the coolant loop 17, 19, 20, 21 can be a forced air liquid-to-air heat exchanger, or a liquid-to-liquid heat exchanger in a hydronic heating system. The heat exchanger 20 can be arranged to provide either forced hot air or radiant heat to a dwelling. The cooling loop can also be replaced or complimented by a domestic hot water tank that will absorb and temporarily store heat that would otherwise be wasted. Coolant 23 may be circulated first through condenser 17 and subsequently through exhaust gas heat exchanger 19, because a greater temperature differential between turbine exhaust 16 and coolant 23 is desirable for efficient condensation of turbine exhaust 16.

CHP systems 200 incorporating the disclosed on demand vapor generators 100 may be configured with the capability of producing heat energy greater than needed for steam production, with the excess heat energy employed for other purposes. For example, if a CHP system is configured with a burner closely matched to the selected expander, there will be modest excess heat collected in the coolant, since most of the heat will be absorbed in the heat exchanger 6. In such an arrangement, the heat recovered in the coolant loop may be insufficient to meet demand for heat, for example to heat a structure on a cold day. Under such circumstances, an additional heat source would be necessary, duplicating much of the structure of the burner 12 and combustion chamber 7 already included in the disclosed systems 100, 200. Providing a more robust heat source permits more widespread use of a CHP, and may reduce or eliminate the need for supplementary heat sources according to aspects of the disclosure. The heat source such as burner 12 and coolant loop components 17, 19, 20, 21 can be designed to allow for the proper amount of heat to be absorbed in the coolant 23 to serve the additional heat demand and eliminate the need for an additional heat source. Designing a single combustion chamber 7 and heat source such as burner 12 that can generate heat energy sufficient to meet the full thermal load for a given installation provides a significant economic advantage by eliminating the equipment cost of the second heating source.

There are also applications where the thermal load is not required and the on-demand vapor generating system 100 is used to perform mechanical work or generate electrical energy. In such an application, the cooling loop will have the same functionality except the heat from the cooling fluid heat exchanger 20 would vent to the atmosphere instead of being used for some other purpose. In such circumstances, it is possible to configure a cooling loop extracting heat from the combustion chamber exhaust at 19 and the expander exhaust at 17 and add some of that heat energy into water delivered to the nozzle 5 in an “economizer” arrangement. Raising the temperature of the working fluid before injection reduces the amount of time and heat energy necessary to transition the water into steam, increasing system efficiency. It may be possible to raise the temperature of the water past its boiling point, in which case at least some of the water would “flash” directly to steam upon passing through the nozzle 5.

The disclosed on demand vapor generator and control system may also be used to harvest waste heat energy from an existing heat source rather than burning a fuel inside a combustion chamber. This method for developing steam can be added to any system that produces waste heat, e.g. large internal combustion generators, kilns, industrial furnaces, etc. The system of FIG. 2 could be modified to work with an existing heat source. The combustion chamber 7 would not have a combustion source such as burner 12 and would not burn any fuel to produce heat. The heat exchanger 7 would be placed in a position (a flue or exhaust pipe) to be heated by exhaust from the original system before it is vented to the atmosphere. These exhaust gases generally carry a large amount of energy, normally wasted, which can be recovered. Approximately 30% of the heat generated by an internal combustion engine is wasted into the exhaust, which also tends to be relatively high temperature. The exhaust of internal combustion engines presents a large opportunity to recovery waste heat according to aspects of the disclosure.

In one disclosed embodiment of a control system, steady state work output is achieved by, but is not limited to, two closed-loop control algorithms operating together as shown in FIG. 9. Both control loops measure system variables and use that feedback to calculate adjustments to system variables to produce the desired steam output, as described above. The control routines may provide feedback to each other by incorporating the output of one control loop with the input or output of the other control loop. One objective of the control loops is to produce the desired work output using a calculated amount of heat input while keeping the steam within the desired range of steam temperature and pressure. The disclosed apparatus and control system improve over the prior art by matching work output with heat energy input, efficiently maintaining a flow of steam in the superheated temperature range while having the capability to quickly change work output as desired. If a change in work is required by the system, the control loops recognize the change in load and set the work output (set point) to the new desired load and maintain that output for the required period of time.

Once steam phase changes to its superheated properties, steam acts as an ideal gas. Following ideal gas laws and keeping volume constant, pressure and/or temperature can be monitored to regulate the mass flow of water according to the equation (PV=nRT). Subroutines 1 and 2 shown in FIG. 9 are linked by the physics of the system. The governing equation for subroutine 1 is: W=Q where W is work output and Q is heat input. The governing equation for subroutine 2 is: Q=m*C_v*(T_2−T_1).

Using the two governing equations of heat transfer and thermodynamics above where heat input, Q, is equal to the mass flow of the water m, multiplied by the specific heat of the water, C_v, multiplied by the change in water temperature. Also where work output, W, is equal to heat input, Q. T2 represents the steam temperature after heat is added to the system and T1 is the temperature of the water before it enters the heat exchanger to phase change to steam. The control program does not calculate the exact amount of heat required; instead the controller may use equations incorporating these principles to continually adjust the mass flow of water and heat input to keep the system at steady state in response to changes in system variables and demand for work (which affects set point). In one control algorithm, the system will set the heat input Q to an initial set point corresponding to a desired work output and then slowly increase the mass flow of water (m). The temperature and pressure of the resulting steam will begin to be read by the controller via signals from sensors 8, 9. As T2 temperature rises past its set point the controller will add more water and T2 will drop as excess heat is absorbed by the injected atomized water. Again as the system reaches steady state T2 will rise. This process will repeat until the steam temperature T2 is stable, meaning that temperature remains within a predetermined range of the steady state set point. Once the temperature stabilizes, the required power will be monitored and if the power output is not at the correct set point, heat will be increased or decreased to accommodate the power demands. This will change the T2 temperature and the control loop illustrated in FIG. 9 will begin stabilizing steam temperature T2 again through the mass flow of water. Although this sounds like a lengthy process, with instant controller feedback times and almost instantaneous computing speeds, this process can be executed rather quickly and steady state flow of steam corresponding to a desired work or power output can be achieved in less than 1 minute.

The equations are used to represent relationships between the variables. They are not accurate for mathematical calculation. When phase changing from water to steam, each phase has a different specific heat. If these equations where used for the phase change from water to steam it would mathematically be incorrect. However these equations can be used to represent the correlations between heat input, work output, temperature and flow, which are the correlations the controller needs to correctly operate.

Embodiments of the disclosed on demand steam generating systems are shown and described for purposes of illustration. The disclosed embodiments are examples and are not intended to limit the scope of the appended claims. Variations of the disclosed structures, functions, and control methods will occur to those skilled in the art, all of which are intended to be encompassed by the claims. 

What is claimed:
 1. A system for generating a variable flow of gaseous phase working fluid, comprising: a heat exchange tube arranged in a combustion chamber, said heat exchange tube having a first end receiving liquid phase working fluid and a second end from which gaseous phase working fluid leaves the heat exchange tube; a variable output heat source arranged to release a variable quantity of heat into said combustion chamber, said variable quantity of heat being a function of power applied to said variable output heat source, wherein said heat is absorbed by said liquid phase working fluid which transitions to a gaseous phase working fluid in said heat exchange tube; a variable flow rate source of liquid phase working fluid arranged to deliver liquid phase working fluid to the first end of said heat exchange tube, a flow rate of said variable flow rate source being a function of power applied to said variable flow rate source; an expander in a gaseous phase working fluid pathway connected to said heat exchange tube second end, said expander configured to use said variable flow of said gaseous phase working fluid at a pre-determined target temperature T to perform work; a temperature sensor arranged to generate a temperature signal TEMP corresponding to the temperature of said gaseous phase working fluid in said gaseous phase working fluid pathway; a controller operatively connected to said variable output heat source, said variable flow rate source of working fluid and said temperature sensor, said controller executing a program, said program comprising: retrieving an initial set point from memory, said initial set point including an initial heat power level and an initial flow rate power level; applying said initial heat power level to said variable output heat source; applying said initial flow rate power level to said variable flow rate source; receiving said temperature signal TEMP; and calculating a correction factor and adjusting one of the power levels, wherein calculating the correction factor and adjusting one of the power levels comprises: calculating a heat correction factor according to the formula Y*ΔT, where Y is a predetermined constant and ΔT is calculated according to T−TEMP, and adjusting the initial heat power level by an amount equal to the heat correction factor to provide an adjusted heat power level and operating said variable output heat source according to the adjusted heat power level while operating said variable flow rate source according to said initial flow rate power level; wherein said steps of operating said variable flow rate source and said variable output heat source, receiving said temperature signal TEMP, calculating the heat correction factor, and adjusting the initial heat power level, are performed in a control loop running continuously until said TEMP stabilizes within a pre-determined range relative to said target temperature T for a pre-determined period of time.
 2. The system of claim 1, wherein said predetermined constant Y has a first value Y1 when T is greater than TEMP and a second value Y2 when T is less than TEMP.
 3. The system of claim 1, wherein power applied to said variable output heat source and said variable flow rate source is varied using pulse width modulation (PWM) and said initial heat power level is a PWM duty cycle applied to said variable output heat source and said initial flow rate power level is a PWM duty cycle applied to said variable flow rate source.
 4. The system of claim 1, wherein said initial flow rate power level produces a flow rate of liquid phase working fluid generating a flow of gaseous phase working fluid from said heat exchange tube second end at said predetermined target temperature T when exposed to a quantity of heat corresponding to said initial heat power level.
 5. The system of claim 1, wherein the liquid phase working fluid is water, said gaseous phase working fluid is superheated steam, said expander is a steam turbine operatively connected to drive an electric generator, and said system further comprises: a condenser fluidly connected to an outlet of said steam turbine, in which steam leaving the steam turbine is cooled and condensed to liquid form, resulting in reduced pressure at the outlet of said steam turbine.
 6. A system for generating a variable flow of gaseous phase working fluid, comprising: a heat exchange tube arranged in a combustion chamber, said heat exchange tube having a first end receiving liquid phase working fluid and a second end from which gaseous phase working fluid leaves the heat exchange tube; a variable output heat source arranged to release a variable quantity of heat into said combustion chamber, said variable quantity of heat being a function of power applied to said variable output heat source, wherein said heat is absorbed by said liquid phase working fluid which transitions to a gaseous phase working fluid in said heat exchange tube; a variable flow rate source of liquid phase working fluid arranged to deliver liquid phase working fluid to the first end of said heat exchange tube, a flow rate of said variable flow rate source being a function of power applied to said variable flow rate source; an expander in a gaseous phase working fluid pathway connected to said heat exchange tube second end, said expander configured to use said variable flow of said gaseous phase working fluid at a pre-determined target temperature T to perform work; a temperature sensor arranged to generate a temperature signal TEMP corresponding to the temperature of said gaseous phase working fluid in said gaseous phase working fluid pathway; a controller operatively connected to said variable output heat source, said variable flow rate source of working fluid and said temperature sensor, said controller executing a program, said program comprising: retrieving an initial set point from memory, said initial set point including an initial heat power level and an initial flow rate power level; applying said initial flow rate power level to said variable flow rate source; applying said initial heat power level to said variable output heat source; receiving said temperature signal TEMP; calculating a heat correction factor according to the formula Y*ΔT, where Y is a predetermined constant and ΔT is calculated according to T−TEMP; adjusting the initial heat power level by an amount equal to the heat correction factor to provide an adjusted heat power level and operating said variable output heat source according to the adjusted heat power level while operating said variable flow rate source according to said initial flow rate power level, wherein said steps of operating said variable flow rate source and said variable output heat source, receiving said temperature signal TEMP, calculating the heat correction factor, and adjusting the initial heat power level, are performed in a control loop running continuously until said TEMP stabilizes within a pre-determined range relative to said target temperature T for a pre-determined period of time.
 7. The system of claim 6, wherein said predetermined constant Y has a first value Y1 when T is greater than TEMP and a second value Y2 when T is less than TEMP.
 8. The system of claim 6, wherein power applied to said variable output heat source and said variable flow rate source is varied using pulse width modulation (PWM) and said initial heat power level is a PWM duty cycle applied to said variable output heat source and said initial flow rate power level is a PWM duty cycle applied to said variable flow rate source.
 9. The system of claim 6, wherein the liquid phase working fluid is water, the gaseous phase working fluid is steam, and the target temperature T corresponds to steam in a superheated temperature range.
 10. The system of claim 6, wherein said expander comprises a turbine coupled to an electric generator.
 11. The system of claim 6, wherein said heat exchange tube is coiled within said combustion chamber.
 12. The system of claim 6, wherein said variable output heat source is a burner arranged to combust fuel within said combustion chamber.
 13. The system of claim 6, wherein the liquid phase working fluid is water, said gaseous phase working fluid is superheated steam, said expander is a steam turbine operatively connected to drive an electric generator, and said system further comprises: a condenser fluidly connected to an outlet of said steam turbine, in which steam leaving the steam turbine is cooled and condensed to liquid form, resulting in reduced pressure at the outlet of said steam turbine.
 14. The system of claim 13, further comprising a closed working fluid path from a reservoir of working fluid, through said heat exchange tube, said steam turbine, said condenser and back to said reservoir of working fluid.
 15. A method of generating a flow of gaseous working fluid comprising the steps of: exposing a heat exchange tube having first and second ends to a variable output heat source, a variable quantity of heat generated by said variable output heat source being proportional to power applied to said variable output heat source; providing a variable flow rate source of liquid phase working fluid in communication with a first end of said heat exchange tube, a flow rate of said variable flow rate source proportional to power applied to said variable flow rate source and said liquid phase working fluid absorbing heat from said variable output heat source which transitions to a gaseous phase working fluid in said heat exchange tube; connecting a gaseous working fluid pathway from said heat exchange tube second end to a turbine driven electric generator; measuring the temperature of the gaseous phase working fluid in said gaseous phase working fluid pathway and generating a temperature signal TEMP corresponding to the temperature of said gaseous phase working fluid in said gaseous phase working fluid pathway; operating said variable output heat source according to an initial heat power level; operating said variable flow rate source of working fluid according to an initial flow rate power level; receiving said temperature signal TEMP; calculating a heat correction factor according to the formula Y*ΔT, where Y is a predetermined constant and ΔT is calculated according to T−TEMP, wherein said predetermined constant Y has a first value Y1 when T is greater than TEMP and a second value Y2 when T is less than TEMP; and adjusting the initial heat power level by an amount equal to the heat correction factor to provide an adjusted heat power level and operating said variable output heat source according to the adjusted heat power level while operating said variable flow rate source according to said initial flow rate power level, wherein said steps of operating said variable flow rate source and said variable output heat source, receiving said temperature signal TEMP, calculating the heat correction factor, and adjusting the initial heat power level, are performed in a control loop running continuously until said TEMP stabilizes within a pre-determined range relative to said target temperature T for a pre-determined period of time.
 16. The method of claim 15, comprising: providing a controller operatively connected to said variable flow rate source, said variable output heat source, and said temperature signal TEMP, said controller including memory and a processor that executes a program including the steps of operating, receiving, calculating and adjusting; deriving a relationship between a power level applied to the variable output heat source and the power level applied to the variable flow rate source by measuring fluid flow needed to keep said temperature signal TEMP within said predetermined range relative to said target temperature T for a given heat output from said variable output heat source at multiple pressures, where the initial flow rate power level produces a flow rate of liquid phase working fluid generating a flow of gaseous phase working fluid from said heat exchange tube second end within said predetermined range of said target temperature T when exposed to a quantity of heat corresponding to said initial heat power level; storing said initial heat power level and said initial flow rate power level in said memory as an initial set point; and in response to a demand for gaseous phase working fluid, retrieving said initial set point from said memory and using said initial heat power level to operate said variable output heat source and said initial flow rate to operate said variable flow rate source.
 17. The method of claim 16, comprising: after said TEMP has stabilized within said predetermined range of said target temperature T for said predetermined period of time, the controller selects a work output set point based upon a demand for gaseous phase working fluid, said work output set point including a steady state heat power level and a steady state flow rate power level corresponding to a flow of gaseous phase working fluid sufficient to meet said demand.
 18. The method of claim 15, wherein said liquid phase working fluid is water and said gaseous phase working fluid is steam, said method comprising: providing a condenser to cool gaseous phase working fluid leaving said turbine, resulting in a flow of water from said condenser; collecting said water in a reservoir, and evacuating said gaseous phase working fluid pathway and said steam turbine by application of negative pressure at an outlet of said turbine.
 19. The method of claim 18, comprising: connecting said reservoir to said variable flow rate source, thereby forming a closed loop in which water is recirculated.
 20. The method of claim 15, wherein said liquid phase working fluid is water, said gaseous phase working fluid is steam, and said target temperature T corresponds to dry superheated steam. 